Case.] Turtles. 383 



The next four — probably the fifth, sixth, seventh, and eighth — 

 have a very thick internal border, rapidly contracting externally 

 to form a sharp edge. The internal surface of each of these is 

 marked by a deep, round pit, which received the end of a rib. 

 The anterior members of the series are represented by two 

 or three short, slender and almost cylindrical elements, which 

 show no trace of articulation with the ribs. The nuchal is 

 a rather thin flattened bone, with a rounded, concave anterior 

 margin. The whole bone is convex from side to side. Its out- 

 line is much the same as in Protostega. There are two large, 

 wing-like lateral expansions, becoming very thin at their pos- 

 terior and lateral margins ; and a posteriorly extending process 

 from the center of the hind margin. Plate lxxxii, figure 3; 

 plate lxxxiii, figure 1. 



The plastron is represented by the hyoplastron, hypoplastron, 

 and xiphiplastron. There is no trace of the entoplastron or the 

 epiplastron. The anterior end of the hypoplastron is still inter- 

 locked and held in position by one of the neurals that has been 

 pressed down upon it from above. The two sides are slightly 

 displaced in the antero-posterior direction. 



The hyoplastron is joined to the hypoplastron by a compara- 

 tively broad area and suture closely resembling the condition in 

 Chelonia. The anterior process extends well forward and in- 

 ward, and interlocks by a strong digital process at its posterior 

 and middle portion with the corresponding bone of the opposite 

 side. At the anterior part the digitations are weaker and more 

 slender. The lateral process does not start from the posterior 

 margin of the bone, but from a point about a centimeter ante- 

 rior to it. It extends outward and a little forward quite to the 

 peripherals, and terminates in strong digitations. On the under- 

 side there is a short, rugose elevation, about a centimeter long, 

 running antero-posteriorly, located near the base of the anterior 

 process. • 



The hypoplastron is quite similar to the hyoplastron in out- 

 line, and of nearly the same size. The posterior process is shorter 

 antero-posteriorly, and extends inward and backward and is in- 

 terlocked with the hypoplastron of the opposite side, as in the 



